In the USA, we usually try to apply the rule that you have to have skin in the game. To vote, you generally must be a properly registered citizen. There are some exceptions for local-issue elections, but to vote for the presidency and Congress, you need to be a citizen.
We do not make the distinction of being smart or stupid as voting criteria. Our symbol of justice is blind - she cannot see that you are a total chowderhead because to us, "all Men are created equal." Therefore, we CANNOT make the claim that person X shouldn't be voting because they are dumber than a box of rusty garden tools. We don't KNOW that the dumb cluck's idea is worse than the genius, often because the issues involved in voting are not based on logic anyway, but on preferences.